Varieties of gases are produced and released during the combustion of coal in the production of electricity. Domestic power plants that release NOx gases are required by Federal and state laws to control the amount of NOx gaseous emissions from their stacks. In order to comply with these laws many plants employ Selective Catalytic Reduction (SCR) systems. SCR systems employ catalysts to enhance the removal of NOx gases from the flue gas as the flue gas passes over the catalyst and reacts with ammonia injected upstream of a SCR reactor to form nitrogen and water vapor. In order for SCR systems to operate effectively the flow of gas over the catalyst should ideally be even and unimpeded.
Large Particle Ash (LPA) or so-called popcorn ash has become a significant issue in the operation and performance of SCR systems on, for example, pulverized coal boilers. Many SCR systems employ catalyst with plate, honeycomb, or corrugated arrangements to maximize catalyst surface area. However, these arrangements often leave the SCR catalysts susceptible to pluggage from LPA generated in some coal fired boilers during plant operation.
It is generally understood that LPA is the result of molten ash (slag) formed in upper furnace and super heater areas and the break-up of ash deposits on tube surfaces of boiler convection passes by soot blowing operation. LPA, if not removed from the flue gas stream, can become lodged within SCR catalyst openings resulting in significant pluggage, erosion and accelerated catalyst deactivation. When LPA blocks the catalyst openings, finer ash particles are prevented from passing through the catalyst and will build up, sometimes in very large amounts, on top of the catalyst. It is not uncommon for piles of ash to reach several feet in height on the top layer of catalyst. With significant amounts of ash buildup, the free flow area of the SCR reactor can be greatly reduced. The flue gas flow is then forced through the remaining open areas of catalyst layer creating higher gas velocities and higher catalyst erosion rates as well as higher pressure drop across catalyst layers. The catalyst underneath the ash piles or the catalyst of which the openings are plugged by LPA is not available for NOx reduction reaction resulting in NOx reduction impairment and higher NOx emission. Excessive ash buildup can also create structural loading concerns as SCR reactors are typically not designed for such large accumulations of ash. All of these issues can ultimately result in forced unit outages for cleaning the ash buildups and the ash pluggage within catalyst openings. Damage to the catalyst from the LPA, however, will require catalyst cleaning, regeneration or replacement. Overall, allowing LPA to reach the SCR reactor can create significant operation and maintenance issues, costly repairs or premature catalyst replacement.
Conventional LPA mitigation systems generally rely on gravity in so-called dropout hoppers often positioned in a horizontal section of ductwork, or in the flue gas stream under an area where ash drop-out normally occurs. These occasionally include baffles in order to deflect the LPA further into the hoppers. The effectiveness of dropout hoppers on LPA capture usually is not completely assured and depends on hopper and adjacent ductwork geometries as well as particle density, particle shape and size distribution of LPA. One drawback to this arrangement is the combination of the low density of LPA and the high gas velocities in the flue gas stream. This combination can lead to LPA that partially dropped out being re-entrained into the stream and continuing onto the SCR reactor.
The second common type of technique is full cross-section screens positioned upstream of a SCR reactor. Often these include flat, pleated or oscillating screens. But whatever the choice of screen, all are affected by high gas velocities in the flue gas stream. The high velocities cause high pressure drop and erosion of the screens. If the screens are plugged by LPA this will increase pressure drop and erosion of unplugged screen areas and result in detrimental effects. If the screen erodes prematurely it will necessitate maintenance and replacement resulting in costly downtime for the boiler.